Part 1 - General information
1) Select the Criteria for design (ex: City of Houston)
This is done by selecting the pulldown list box with your mouse.
2) Select your design storm (ex: 2-year)
3) Fill in project detail information.
4) Fill in area names for your drainage areas.
(notice as you fill in the area names the program autofills the inlet names.)
5) Fill in the respective drainage area sizes (in acres)
6) Fill in the runoff coefficient for each area.
7) Fill in the Manning 'n' value for each area
8) Fill in the Length of each run of pipe
9) Fill in the Grade (ft/100ft) each pipe will be laid at.
10) Fill in the down stream flowline for the last pipe in each grouping.
11) Fill in the Inlet elevation for each inlet
12) Fill in the proposed ground elevation Upstream and Downstream for each pipe
13) Fill in the K factor for each junction.
Part 2 - Link it all together
Each lead coming into the Main Trunk or another lead must be linked in order to link the lead to the main trunk you need to add the Total Area Column for the lead to the Total area Column where the flow is entering the trunk. The downstream flowline of the lead needs to be linked in the same manner.

Calculation Reference
Storm water
Sewer design
Rainwater engineering

To calculate stormwater runoff and appropriately size a drain, you can use the Rational Method. This method is based on the assumption that the rate of runoff (Q) is proportional to rainfall intensity (i), drainage area (A), and a runoff coefficient (C) that represents the portion of rainfall that becomes runoff. Here's the formula:

Q = CiA

where: Q = Runoff rate (cubic feet per second, or CFS) C = Runoff coefficient (dimensionless) i = Rainfall intensity (inches per hour, or in/hr) A = Drainage area (acres)

To use the Rational Method, follow these steps:

  1. Determine the drainage area (A): Measure or estimate the area that contributes runoff to the drain. This can be done using site plans, aerial imagery, or topographic maps.

  2. Select the appropriate runoff coefficient (C): The runoff coefficient represents the fraction of rainfall that becomes runoff. It depends on factors such as surface type, soil type, and slope. Here are some typical values:

  • Pavement or concrete: 0.9 - 0.95
  • Asphalt: 0.85 - 0.9
  • Gravel: 0.7 - 0.8
  • Lawns (flat): 0.1 - 0.3
  • Lawns (moderate slope): 0.2 - 0.4
  • Lawns (steep slope): 0.3 - 0.6
  1. Determine the rainfall intensity (i): You'll need to refer to the local rainfall data and choose an appropriate design storm for your area. This is usually represented as a return period (e.g., 10-year, 25-year, or 100-year storm) and can be found in the local stormwater management guidelines or on the websites of national weather agencies.

  2. Calculate the runoff rate (Q): Plug your values for C, i, and A into the Rational Method formula to calculate the runoff rate (Q).

  3. Size the drain: The drain should be sized to handle the calculated runoff rate (Q). You can consult manufacturer's specifications for drain capacity or consult with a professional engineer to ensure that the selected drain is adequate for the intended application.

Keep in mind that local regulations and guidelines may require more complex calculations or different methods. Consult with a professional engineer or stormwater management expert to ensure that your design complies with all relevant regulations.

Calculation Preview

07 Dec 2009
Last Modified
27 Apr 2023
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Comments: 1
infostar 3 years ago
Thanks for creating the tool. While using the tool, I have noticed that the time of concentration formula seems to be not working. Perhaps, I am missing something. Please let me know, if you have detailed instructions to follow or an updated version of this tool that you can share. Thanks, Aravind